genetlink: fix error return code in genl_register_family()

Fix to return a negative error code from the idr_alloc() error handling
case instead of 0, as done elsewhere in this function.

Also fix the return value check of idr_alloc() since idr_alloc return
negative errors on failure, not zero.

Fixes: 2ae0f17d

 ("genetlink: use idr to track families")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>